Hot water systems are generally composed of all-ferrous metallurgy or mixed metallurgy, such as copper or copper alloy systems, nickel and nickel-based alloys, and stainless steel and may also be mixed with mild steel components. Many general classes/components of hot water systems exist, such as boilers, hot water heaters, heat exchangers, steam generators, steam jackets and molds, nuclear power electric systems, combustion engine and diesel coolant systems, evaporator systems, thermal desalination systems, papermaking operations, fermentation processes, the like, and attached ancillary devices. They are dynamic operating systems that undergo a myriad of REDOX Stress events (i.e., any electrochemical event in the hot water system related to changes in oxidative or reductive state of the water). Such events generally include any process that implicates the oxidation-reduction potential (“ORP”) space or regime in the system.
These events may result from a multitude of factors including species inherently found in the water, leaks from various components, contamination from air in-leakage, malfunctioning pumps, seals, vacuum lines, and gauges. Further, increased use of oxygen-enriched water, such as boiler make-up water, returned steam condensate, and/or raw surface or subsurface water, deaerator malfunctions, steam and turbine load swings, and problems with chemical feed pumps cause unplanned reduction or increase in chemical treatment feed rates. Uncontrolled REDOX Stress events can cause serious general and localized corrosion problems, such as pitting, stress corrosion cracking, corrosion fatigue, and/or flow accelerated corrosion problems in hot water systems. By their nature, these problems tend to be electrochemical and thus tied into the oxidative-reductive properties of the environment and the structural material interaction.
Additional problems are encountered when plant equipment undergoes intermittent operations, including using steam in a periodic fashion. For example, a steam operated mold press in tire manufacturing or components of a papermaking process may be in that category. In the extreme, a steam system might be put into a layup sequence to either be idled (or slowed down) or started from idle to operating conditions. A main concern is corrosion that occurs in situ followed by corrosion product transport to other plant locations during startup or shutdown of the intermittent process. Apart from the localized damage caused by in situ corrosion, the corrosion product transport can foul a multitude of components. Typically, the worst fouling occurs on the boiler tube walls where deposits can lead to poor heat transfer, increased boiler tube temperatures, under-deposit corrosion, local overheating, and tube failure. In situ corrosion and corrosion product transfer may also impact the balance of the entire system, not just the boiler/steam generator. Impacted components may include transfer piping, holding tanks, and ancillary equipment that come into contact with the hot water/steam process fluids.
Although some conventional methods are practiced today to identify REDOX Stress events in hot water systems, because of hot water system dynamics most REDOX Stress events are unpredictable. These methods are not widely practiced because they have inherent drawbacks (see below). As a consequence, the majority of REDOX Stress events go undetected and thus uncorrected. Uncontrolled REDOX Stress events can lead to serious corrosion problems in these systems, which negatively impact plant equipment life expectancy, reliability, production capability, safety, environmental regulations, capital outlay, and total plant operation costs.
Identifying REDOX Stress events currently includes both online instruments and grab sample wet chemical analysis test methods. In both approaches, the sample has to first undergo sample conditioning, such as cooling, prior to measurement. Examples of online instruments include dissolved oxygen meters, cation conductivity instruments, room temperature ORP instruments, pH instruments, sodium analyzers, hardness analyzers, specific conductivity meters, silica analyzers, particle and turbidity meters, reductant analyzers, and the like. General corrosion monitoring, such as coupon and electrochemical analysis, is typically performed after cooling a sample or at elevated temperatures. Grab sample test methods include analyzing for dissolved oxygen, pH, hardness, silica conductivity, total and soluble iron, copper, and silica, reductant excess, and the like.
Some drawbacks of these methods include the following. Grab sample analysis gives a single point in time measurement and consequently is not a viable continuous monitoring method for REDOX Stress events. It also often has inadequately low-level detection limits. Online monitors do not provide a direct measurement of REDOX Stress and thus cannot indicate whether or not a REDOX Stress event is occurring at any particular time. Corrosion monitors detect general corrosion, but are not capable of measuring changes in local corrosion rates caused by REDOX Stress events. Online reductant analyzers measure the amount of reductant, but not the net REDOX Stress a system is undergoing at system temperature and pressure. That REDOX Stress can occur in the apparent presence of a reductant is thus another drawback of this technique.
Dissolved oxygen (“DO”) meters have similar drawbacks. Measuring the amount of DO (an oxidant) but not necessarily the net REDOX Stress a system is undergoing is not an accurate indicator of corrosion stress. The sample also must be cooled prior to DO measurement thus increasing the lag time in detecting when the REDOX Stress event started. Further, the potential for oxygen consumption in the sample line could cause inaccurate readings. REDOX Stress can also occur in the apparent absence of DO and little or no DO in the sample could potentially be a false negative. In addition, all of the instruments described above are relatively costly to purchase, and require frequent calibration and maintenance.
Corrosion coupons give a time-averaged result of general system corrosion. Again, this technique does not offer a real-time indication or control of REDOX Stress events. Online electrochemical corrosion tools are inadequate for localized corrosion determinations and cannot be used in low conductivity environments. Integrated metal sampling techniques that use filtration to obtain a water sample on a cooled sample stream also provide no active real-time response with respect to REDOX Stress events.
Room temperature ORP is a direct measurement of the net ORP of a sample taken from the system. A drawback of this technique is that it fails to indicate what is happening at system temperature and pressure. REDOX Stress events that occur at operating temperature and pressure often cannot be observed at room temperature, as process kinetics and thermodynamics vary with temperature. In addition, room temperature ORP measuring devices are more sluggish and more likely to become polarized. Reliability of such devices is poor and they need frequent calibration and maintenance.
There thus exists an ongoing need to develop methods of accurately monitoring and controlling real-time ORP in hot water systems to minimize corrosion, particularly while undergoing a layup sequence.